# mod_11_2.py - functions for performing the ISO 7064 Mod 11, 2 algorithm
#
# Copyright (C) 2010 Arthur de Jong
#
# This library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
# License as published by the Free Software Foundation; either
# version 2.1 of the License, or (at your option) any later version.
#
# This library is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with this library; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
# 02110-1301 USA

"""Module for calculation and verifying the checksum of a number
using the ISO 7064 Mod 11, 2 algorithm.

Validation can be done with is_valid(). A valid number can be made by
calculating the check digit and appending it.

>>> calc_check_digit('0794')
'0'
>>> is_valid('07940')
True
>>> calc_check_digit('079')
'X'
>>> is_valid('079X')
True
>>> checksum('079X')
1
"""


def checksum(number):
    """Calculate the checksum."""
    check = 0
    for n in number:
        check = ( 2 * check + int(10 if n == 'X' else n) ) % 11
    return check

def calc_check_digit(number):
    """With the provided number, calculate the extra digit that should be
    appended to make it a valid number."""
    c = (1 - 2 * checksum(number) ) % 11
    return 'X' if c == 10 else str(c)

def is_valid(number):
    """Checks whether the check digit is valid."""
    try:
        return bool(number) and checksum(number) == 1
    except:
        return False